It is often desirable to be able to make two measurements and to remove the common mode component of those measurements to look at a difference between the measurements. By way of example, consider a relatively high voltage supply line providing power to a load. It may be desirable to measure the current provided to the load, and this can be done by inserting a low value sense resistor, Rsense, in series with the load. The voltage at the supply side of the resistor will be the supply voltage VHT whereas the voltage on the load side of the sensing resistor will be VHT−(Rsense×IHT) where IHT represents the current flowing through the resistor. VHT may be many tens or hundreds of volts while the voltage dropped across the sensing resistor Rsense may only be in the region of millivolts.
It is often desirable to amplify the voltage drop across the sensing resistor Rsense and then provide this to other circuits where this value can be displayed or used. However the amplifiers and subsequent components often work at relatively low voltages, for example five or less volts, referenced to a ground potential. The semiconductor process used to form these components is often unsuitable for high voltages and frequently incapable of withstanding them without damage.
It is known that the voltages occurring across the sense resistor Rsense could be attenuated by resistor potential dividers such that the voltage at the output of the potential divider is suitable for provision to an amplifier or other signal processing circuitry using relatively low voltage transistors. However the potential divider attenuates the common mode voltage and the differential voltage equally and the resistors used in the potential divider act as noise sources.